<template>
    <ul class="alphabet">
        <li class="item" 
            v-for="item of letters" 
            :key="item"
            :ref="item"
            @click="handleLetterClick" 
            @touchstart.prevent="handleTouchStart"
            @touchmove="handleTouchMove"
            @touchEnd="handleTouchEnd"
        >{{ item }}</li>
    </ul>
</template>

<script>
export default {
    name: 'CityAlphabet',
    props: {
        cities: Object
    },
    data(){
        return{
            touchStatus: false,
            startY: 0,
            timer: null
        }
    },
    updated(){  //数据发送更新时
        this.startY = this.$refs['A'][0].offsetTop
    },
    computed: {
        letters(){
            const letters = []
            for(let i in this.cities){
                letters.push(i)
            }
            return letters
        },
        // startY(){
        //     return this.$refs['A'][0].offsetTop
        // }
    },
    methods: {
        handleLetterClick(e){
            this.$emit('change',e.target.innerText)
        },
        handleTouchStart(){
            this.touchStatus = true
        },
        handleTouchMove(e){
            if(this.touchStatus){
                if(this.timer){  //函数节流
                    clearTimeout(this.timer)
                }
                this.time = setTimeout(()=>{
                    const touchY = e.touches[0].clientY - 79
                    const index = Math.floor((touchY - this.startY)/20)
                    if(index >= 0 && index < this.letters.length){
                        this.$emit('change',this.letters[index])
                    }
                },16)
            }
        },
        handleTouchEnd(){
            this.touchStatus = false
        }
    }
}
</script>

<style lang="stylus" scoped>
    @import '~styles/varibles.styl'

    .alphabet
        display flex
        flex-direction column
        justify-content center
        position absolute
        top 1.58rem
        right 0
        bottom 0
        width .4rem
        .item
            line-height .4rem
            text-align center
            color $bgColor
</style>

